1. Field of the Invention
This invention relates to a charging member for charging a member to be charged. The invention also relates to an image forming apparatus such as a copying machine or a printer in which a charging member is brought into contact with a member to be charged to thereby charge the surface of the member to be charged.
2. Related Background Art
Heretofore, a contact charging apparatus has been one in which an electrically conductive charging member (a contact charging member or a contact charger) of a roller type (charging roller), a fur brush type, a magnetic brush type, a blade type or the like is brought into contact with a member to be charged such as an image bearing member, and a predetermined charging bias is applied to this contact charging member to thereby charge the surface of the member to be charged to a predetermined polarity and potential.
These charging apparatuses are expressed together as a contact charging apparatus, but the individual charging apparatuses differ greatly from one another in the viewpoint of the charging mechanism (the mechanism of charging and the principle of charging) thereof. In the charging mechanism of contact charging, there exist I. a discharging type charging mechanism and II. a direct injecting type charging mechanism. The feature of the charging apparatus is determined depending on by which charging mechanism the charging apparatus is. The principles and features of the discharging type charging mechanism and the direct injecting type mechanism will hereinafter be described.
I. Discharging Type Charging Mechanism
This is a mechanism in which the surface of a member to be charged is charged by a discharge product by a discharging phenomenon occurring in the gap between a contact charging member and the member to be charged.
A discharging type charging system has a constant discharging threshold value in the contact charging member and the member to be charged and therefore, as shown by A (a conventional type roller charging apparatus) in FIG. 5 of the accompanying drawings, it is necessary to apply a voltage greater than the potential of the member to be charged to the contact charging member. Also, as compared with a corona charger, a discharge product is created in principle though marked small in the amount created.
A roller charging process (roller charging apparatus) using an electrically conductive roller (charging roller) as the contact charging member by discharge is preferable in respect of the stability of discharge, and is widely used.
This charging roller for discharge is made by forming a rubber material or a foamed material of electrical conductivity or medium resistance into a roller shape as a base layer, and covering the surface thereof with a high resistance layer. In this construction, discharging phenomenon occurs in a gap with several of tens μm a little distance from the portion of contact between the roller and the member to be charged. Accordingly, in order to stabilize the discharging phenomenon, the surface layer of the roller is flat and the average roughness Ra of the surface is sub-μm or less, and the surface has high roller hardness.
Also, the roller charging by discharging is high in applied voltage and if there is a pinhole (the exposure of a substrate by the injury of the film of the member to be charged), a voltage drop will spread to even the periphery thereof and faulty charging will occur. Accordingly, the surface resistance of the surface layer is made equal to or greater than 1011Ω to thereby prevent the voltage drop.
II. Direct Injecting Type Charging Mechanism
Direct injecting type charging is a charging mechanism in which the exchange of charges is directly done by the contact at a molecular level between the contact charging member and the member to be charged to thereby charge (electrify) the surface of the member to be charged. It is referred to also as direct type charging or injecting type charging.
In this charging mechanism, the potential difference between the contact charging member and the member to be charged is of the order of several V to several tens of V. The charging characteristic thereof is shown by B (magnetic brush charging apparatus) in FIG. 5. The charging potential is equal to an applied voltage, and there is no applied voltage difference causing discharge. Also, the voltage necessary for charging is suppressed to a low level.
As described above, this direct type charging system as a charging mechanism does not result in the production of ions and therefore does not cause any evil by a discharge product. That is, it is a charging process excellent in terms of the safety of environment, the deterioration of the member and low electric power.
Description will now be made of a charging apparatus by the direct injecting type charging mechanism.
In the direct type charging mechanism, an important factor which determines charging performance is the contacting property between the contact charging member and the member to be charged. The contacting property herein referred to means the performance of the contact type charging member being capable of microscopically contact with how much of the surface of the member to be charged while the latter passes through the charging apparatus.
As a form of the contact charging member used in the direct injecting type charging apparatus, an attempt by a charging roller for discharging or the like has been made, but direct injecting type charging has been impossible by the charging roller for discharging. This is because in the high-hardness and smooth surface structure as previously described, the contact charging member appears to be in close contact with the member to be charged, but is scarcely in contact with the latter in the sense of a microscopic contacting property at a molecular level necessary for charge injection.
As a direct injecting type charging process proposed at present, there is particle charging using a magnetic brush.
Thinking of improvements in particle charging and contact density, a charging process (particle charging) using electrically conductive particles is advantageous. The electrically conductive particles used at this time are referred to as the “charging particles”. As examples of an apparatus of a charging type using the charging particles, there have been proposed A. a magnetic brush charging apparatus using a magnetic brush charging member having magnetically restrained electrically conductive magnetic particles as the charging particles as a brush by a magnet, and B. a charging apparatus using a charging member having a thin electrically conductive particle layer formed on an elastic roller.
A. Magnetic Brush Charging Apparatus
FIG. 6 of the accompanying drawings is a model view schematically showing the construction of an example of the magnetic brush charging apparatus 100. The reference numeral 120 designates a magnetic brush charging member comprising a fixedly supported magnet roll 122, including magnetic poles N1, N2, S1, and S2, a nonmagnetic and electrically conductive charging sleeve 121 rotatably fitted around and concentrically with the magnet roll 122, and a magnetic brush layer (magnetic brush portion) 124 of electrically conductive magnetic particles C formed while being attracted to and held on the outer peripheral surface of the charging sleeve 121 by the magnetic force of the magnetic roll 122 in the charging sleeve. The reference numeral 123 denotes a casing to which the magnetic brush charging member 120 is assembled and in which a suitable amount of electrically conductive magnetic particles C is contained and stored. The reference numeral 125 designates a magnetic brush layer thickness regulating blade provided in the casing 123.
As the electrically conductive magnetic particles C which are charging particles causing the magnetic brush layer 124 to be constituted, use is made of magnetic metal particles such as ferrite or magnetite or these magnetic particles bound by resin. The resistance value thereof is 1×106 to 109 Ωcm. The particle diameter thereof is 10 to 50 μm.
The charging sleeve 121 is rotatively driven in the same clockwise direction of the arrow as e.g. a photosensitive drum 1 as a member to be charged. The magnetic brush layer 124 is rotatively conveyed in a clockwise direction with the charging sleeve 121, and is regulated to a predetermined layer thickness by the blade 125, and the layer-thickness-regulated magnetic brush layer 124 contacts with the photosensitive drum 1 and rubs against the surface of the photosensitive drum 1 in a charging contact portion n. The magnetic brush layer 124 having passed through the charging contact portion n is return-conveyed to an electrically conductive magnetic particle reservoir portion in the casing 123 by the continued rotation of the charging sleeve 121, and is circularly conveyed and used.
A predetermined charging bias is applied from a charging bias applying voltage source V1 to the charging sleeve 121, and the surface of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential in the charging contact portion n by a direct injecting type charging mechanism with the aid of the rubbing by the magnetic brush layer 124 and the applied charging bias.
B. Charging Apparatus by Thin Layer Electrically Conductive Particles
FIG. 7 of the accompanying drawings is a model view schematically showing the construction of an example of a charging apparatus 20 by thin layer electrically conductive particles. This charging apparatus 20 has a charging roller 2 as a contact charging member, a charging bias applying voltage source S1 for the charging roller, and a charging particle supplying device 3.
The charging roller 2 comprises a mandrel 2a and an elastic medium-resistance layer 2b of rubber or a foamed material as a charging particle bearing member formed into a roller shape concentrically and integrally with the outer periphery of the mandrel 2a, and further has a thin layer of charging particles (electrically conductive particles) m borne on the outer peripheral surface of the elastic medium-resistance 2b. 
This charging roller 2 is pressed into contact with the photosensitive drum 1 as the member to be charged with a predetermined amount of entry to thereby form a charging contact portion n of a predetermined width. The charging particles m borne on the charging roller 2 contact with the photosensitive drum 1 in the charging contact portion n.
The charging roller 2 is rotatively driven in the same clockwise direction of the arrow as the photosensitive drum 1, and is rotated in a direction opposite to the direction (counter-clockwise) of rotation of the photosensitive drum 1 in the charging contact portion n, whereby it contacts with the surface of the photosensitive drum 1 with a speed difference with the charging particles m interposed therebetween.
The relative speed difference of the charging roller 2 relative to the photosensitive drum 1 can be provided by rotatively driving the photosensitive drum in a direction counter to the direction of rotation of the charging roller 2 (a direction of rotation forward to the rotation of the photosensitive drum 1) at a different peripheral speed. The charging property of direct injecting type charging, however, depends on the ratio between the peripheral speed of the photosensitive drum 1 and the peripheral speed of the charging roller 2 and therefore, it is more advantageous in respect of the number of revolutions to rotatively drive the charging roller 2 in the same direction as the photosensitive drum 1, and it is also preferable in respect of the retain ability of the particles to adopt this construction.
During the image recording by an image recording apparatus, a predetermined charging bias is applied from a charging bias applying voltage source S1 to the mandrel 2a of the charging roller 2.
Thereby, the peripheral surface of the photosensitive drum 1 is uniformly contact-charged to a predetermined polarity and potential by a direct injecting type charging process.
The charging particles m applied to the outer peripheral surface of the charging roller 2 adhere to and are taken away by the surface of the photosensitive drum 1 with the charging of the photosensitive drum 1 by the charging roller 2. Accordingly, in order to make up for it, a charging particle supplying device 3 for the charging roller 2 is required. The application of the charging particles m to the charging roller 2 by the charging particle supplying device 3 is effected by agitating the charging particles m stored in the housing container 3a of the charging particle supplying device 3 by an agitating vane 3b and supplying them to the outer peripheral surface of the charging roller 2. Any charging particles m which become excessive in conformity with a target amount of application are scraped off by a fur brush 3c to thereby effect the application of a proper amount of charging particles. The control of the amount of application of the charging particles is adjustable at any time by the control of the number of revolutions of the fur brush 3c. 
C. Aptitude of Particle Charging to a Cleanerless System
Particle charging is suitable for the toner recycle system of an image forming apparatus. That is, a toner recycle process is an excellent construction in a transfer type image recording apparatus wherein waste toner (untransferred toner) is used again for image forming to thereby effectively make the most of the toner and eliminate a space for a cleaner container and realize the downsizing of the image recording apparatus.
The untransferred toner is once introduced into a contact charging member and is made ready for reuse (the original amount of charge of the toner) and is returned to a developing apparatus through an image bearing member and is used again for developing, or if unnecessary, is collected, whereby, toner recycle becomes possible. A charging apparatus used here is required to charge the image bearing member and in addition, to collect the untransferred toner and recharge the toner.
From the viewpoint as described above, an attempt is made to think of the aptitude of particle charging to toner recycle. A magnetic brush has the features that itself is comprised of particles and can move with a degree of freedom, and is great in contact area. Accordingly, in the magnetic brush, it becomes possible to advantageously realize such functions requisite in toner recycle as collecting the untransferred toner from on the image bearing member, and further making the charges of the introduced toner proper.
In the conventional charging technique as described above, however, it has become apparent that the image recording apparatus causes the following deterioration of the quality of image. Firstly, the problem of the uniformity of a halftone image. When a uniform image of a medium density area has been outputted, the has been a black streaked faulty image like a trace swept by a broom, and also in a halftone image, there has occurred a white spot-like faulty image of the order of 0.1 to 0.5 mm. Further, there has occurred a faulty image having its ground slightly developed, i.e., fog. Observing the state of the fog well, it has been characteristic that the fog toner is distributed with a certain unit. Particularly these are remarkable in the lowering of performance under a high-temperature high-humidity environment. Also, they have been remarkable in a printing test after the image recording apparatus has been left as it is for a long period.